The coating of electroconductive substrates by electrodeposition is an important industrial process. In this process, a conductive article is immersed as one electrode in a coating composition made from an aqueous dispersion of film-forming polymer. An electric current is passed between the article and a counter electrode in electrical contact with the aqueous dispersion until a desired amount of coating is produced on the article. The article to be coated can be made the anode or the cathode depending upon the ionic nature of the coating system.
Cationic coating compositions generally are derived from resinous compositions containing a basic nitrogen atom which can be neutralized with an acid and then be dissolved or dispersed in water. Sufficient basic nitrogen atoms should be present so that the dispersibility or solubility can be obtained with a minimum amount of acid.
The most commonly employed type of cathodically electrodepositable resins are made by reacting polyepoxide resins with amines at elevated temperatures, as exemplified by U.S. Pat. No. 4,137,140 to Belanger and U.S. Pat. No. 4,182,831 to Hicks. Belanger teaches forming an electrocoat resin by reacting polyepoxides with polyamines and then modifying the product by reaction with a monoepoxide or a monocarboxylic acid. In the Hicks patent, the electrocoat resin is taught to be the reaction product of polyepoxides, a mixture of primary amines, and a monoepoxide. As illustrated by these patents, when forming this type of resin the amount of amine reacted with the epoxy group containing material is generally at least that amount necessary to react all the epoxide groups and form a hydroxyl amine resin. However, since these resins contain essentially no unreacted epoxide groups available for later crosslinking the amine during curing, they require a crosslinking agent which is capable of reacting with the hydroxyl or amine functionality of the resin during curing to form a thermoset film. The crosslinking agent may be present in the coating bath so as to codeposit with the resin or it may be incorporated into the resin molecule. Hicks and Belanger teach codepositing the resin with a crosslinker such as an aminoplast or phenoplast resin. On the other hand, Jerabek et al in U.S. Pat. Nos. 3,922,253 and 3,947,338 disclose reacting a partially blocked isocyanate crosslinking agent with the epoxy resins and amines, so as to incorporate the crosslinker into the resin molecule. That electrocoat resin product is thus able to self-crosslink during baking to form a thermoset film.
Binders for cathodic electrodeposition have been prepared by simply combining epoxy resins with amine compounds. However, aqueous dispersions of these binders are very unstable, because of the presence of free (unreacted) epoxide groups. Thus, these two component compositions are less than desirable for use as electrodepositable coatings. Such compositions are taught by Munn et al in British Pat. No. 1,235,975 and in Room Temperature Curing Electrodeposited Coatings, A. G. North, J. Oil Colour Chem. Assoc., 53 (1970) 353. While it is generally recognized in these references that the stability of the compositions in the coating bath is usually limited to a few hours, one advantage of this type of coating, when compared to prior art electrodepositable coatings, is taught to be its ability to be cured at low temperatures, e.g., room temperature.